1. Field of the Invention
The invention concerns a method of incinerating combustible materials, especially urban waste, producing solids and gases which are substantially harmless to the environment. In this method the combustible materials are introduced into a controlled feed furnace and burned in contact with a flow of primary air. Combustion gases are emitted containing particulate unburned materials and gasified fractions whereas non-volatile fractions are consumed thus producing clinker. Secondary air is added to the combustion gases emitted to oxidize the particulate unburned materials and combustible gasified fractions thus generating flue gases and fly ash. The flue gases are passed through a heat exchanger in order to extract heat from them and then into a cleansing area in which the fly ash is separated out and collected and harmful gases are neutralized.
2. Description of the Prior Art
At present, in France, almost 40% of urban waste is incinerated. The process has been developed to a very high technical level, with a combustion efficiency of 98% to 99%, concentrations of unburnt materials that can be as low as 1%, energy efficiencies of 80% to 85% and installation availabilities of 85% to 90%.
At the same time, emission of pollutants has been significantly reduced. Despite such improvements, incineration plant has the reputation of being a source of pollution, possibly because the reduction in emission of pollutants has gone hand in hand with increases in plant size and in the quantities of material processed
What is more, parallel methods of eliminating these waste materials do not seem to be likely to supplant incineration: landfill will be drastically curtailed in the future, being authorized only for harmless residues from other processes; sorting techniques such as selective collection can be applied only to recoverable waste, representing at best 15% to 20% of the total amount of urban waste; in the best case scenario, composting could handle only a maximum of 45% of the waste collected, and represents no advance over conventional incineration in the case of some pollutants such as heavy metals.
It follows that reducing the pollution associated with the elimination of urban waste cannot be achieved by abandoning incineration and this implies that incineration methods need to be improved so that their byproducts are harmless to the environment.
These byproducts are essentially of two kinds, those discharged into the atmosphere in the form of gas and vapor (airborne) and those of solid form such as clinker and fly ash.
The former comprise mostly nitrogen, carbon dioxide and water vapor, with a small proportion of pollutant gas and vapor. Some of these pollutants are already eliminated in large part by appropriate processing:
particulate heavy metals and dust; PA1 hydrogen chloride gas; PA1 carbon monoxide and gaseous hydrocarbons; PA1 volatile heavy metals (mercury). PA1 Current treatment methods are open to improvement. PA1 to complete consumption of said clinker it is decanted into a brazier area with the terminal section equipped with heat energy injector means adapted to fuse the clinker before it is removed through a water seal in which it is extinguished; PA1 said combustion gases are injected with said secondary air into a fluidized bed secondary furnace; PA1 the fly ash leaving said secondary furnace with said flue gases is collected at at least one point on the upstream side of said cleansing area; and PA1 the fly ash collected at said point of the upstream side of said cleansing area and in the latter is recycled into said brazier area on the entry side of said terminal section.
Other gaseous pollutants require action (nitrogen oxides, sulfur compounds, dioxins and other organic compounds). Some of these are unavoidable (dust, volatile heavy metals, sulfur oxides); others are the result of combustion conditions (nitrogen oxides NO.sub.x, dioxins, CO and various organic compounds); in the case of sulfur compounds the combustion conditions can vary the relative proportions of SH.sub.2, SO.sub.2 and SO.sub.3.
The solid byproducts comprise clinker and the byproducts of flue gas cleansing; they have usually been regarded as harmless to the environment. Following removal of metal debris, clinker has often been used in civil engineering works, for example.
However, recent studies have shown that when exposed to rainwater the solid byproducts can discharge soluble fractions including chlorides, sulfates and heavy metals, in concentrations that could be regarded as excessive in the foreseeable future.
The object of the present invention is to reduce airborne and solid pollutants to a level compatible with foreseeable regulations one decade from now.